FIG. 4 is a sectional view showing a schematic structure of a polymer network liquid crystal 10. In the figure, symbols 11 and 12 each denote a glass substrate, symbols 13, 13, . . . each denote a segment electrode that is a transparent electrode formed on the glass substrate 11, symbol 14 denotes a common electrode that is a transparent electrode formed on the glass substrate 12, and symbol 15 denotes a polymer network liquid crystal layer enclosed between the glass substrates 11 and 12.
When no signals are applied to the segment electrodes 13, 13, . . . , i.e., in an OFF state, liquid-crystal molecules of the polymer network liquid crystal layer 15 are irregularly oriented over the pixel region between each segment electrode 13 and the common electrode 14, causing transmitted light to scatter and yielding an opaque appearance.
On the other hand, when signals are applied to the segment electrode 13, i.e., in an ON state, the liquid-crystal molecules of the polymer network liquid crystal layer 15 are aligned in the direction of an electric field in the pixel region between this segment electrode 13 and the common electrode 14, permitting practically all the transmitted light to pass through and yielding a transparent appearance.
FIG. 5 shows a fundamental configuration of a drive circuit for the segment electrodes 13, 13, . . . and the common electrode 14. A power source PS supplies power, which is provided via switches SWc, SWs1, . . . to the common electrode 14 and the segment electrodes 13, 13, . . . in parallel. The switches SWc, SWs1, . . . are adapted to switch a normal/inverted waveform of the power provided to the common electrode 14 and the segment electrodes 13, 13, . . . by their continued operations, respectively.
FIG. 6 exemplifies the voltage waveforms applied to the common electrode 14 and the given segment electrode 13. FIG. 6 (A) shows the instance where the segment electrode 13 is ON, and FIG. 6 (B) shows the instance where the segment electrode 13 is OFF.
As shown in FIG. 6 (A-1), the common electrode 14 is applied with, for example, rectangular waves having a frequency f=32 [Hz] or so and a wave height Vop. To turn on the segment electrode 13, a voltage that corresponds to the inverted version of the rectangular waves applied to the common electrode 14 is applied to the segment electrode 13, as shown in FIG. 6 (A-2).
This causes the polymer network liquid crystal layer 15 in the pixel region between the segment electrode 13 and the common electrode 14 to be applied with a voltage that has a waveform similar to the voltage applied to the common electrode 14 and a doubled wave height 2Vop ranging from voltage −Vop to +Vop, as shown in FIG. 6 (A-3).
On the other hand, in the instance of FIG. 6 (B) where the segment electrode 13 is OFF, the common electrode 14 is applied with a voltage of rectangular waves as shown in FIG. 6 (B-1), while the segment electrode 13 is applied with a voltage of rectangular waves similar to the rectangular waves applied to the common electrode 14 as shown in FIG. 6 (B-2).
Accordingly, the waveform of the voltage applied to the polymer network liquid crystal layer 15 in the pixel region between the segment electrode 13 and the common electrode 14 becomes flat at the GND level as shown in FIG. 6 (B-3), resulting in no electric field between the electrodes and turning the polymer network liquid crystal layer 15 opaque as discussed above.
The main factor in power consumption of the polymer network liquid crystal 10 is a charge-and-discharge current that follows the capacitance between the common electrode 14 and the segment electrodes 13, 13, . . . when the segment electrodes 13, 13, . . . are turned ON.
A concrete consumption current when the segment electrode 13 is turned ON will be explained using FIG. 7.
FIG. 7 (A) is a voltage waveform applied to the common electrode 14, and FIG. 7 (B) is a voltage waveform applied to the segment electrode 13. FIG. 7 (C) shows the transfer of electric charges in a pair of the common electrode 14 and the segment electrode 13.
As shown in this figure (C), electric charges of as much as 2 Q, i.e., −Q to +Q, move from the power source PS to the pixel electrode of the liquid crystal 10 twice at a timing t1 and a timing t2 during one cycle 1/f[sec].
Therefore, the consumption current I is given as:I=d(4Q)/dt  (1)
Assuming that the capacitance between the common electrode and the segment electrode is C, Q is given as:Q=C*Vop  (2)
From the expressions (1) and (2), the consumption current I is:I=4f*C*Vop  (3)
There is a demand for the technique to reduce this consumption current I, further from the content shown by the expression (3).
In this relation, Patent Literature 1 discloses a technique for performing halftone display under a wide range of temperature in a polymer dispersed liquid crystal (PDLC).